The present invention relates to defibrillating and monitoring systems and more particularly to monitoring systems having selective signal inputs by either the defibrillation leads or the conventional patient leads.
Combined heart monitoring and defibrillation systems are known in which a single set of electrodes is utilized both as the ECG pick-up electrodes and as the defibrillation electrodes. With such electrode arrangements, a patient may be first monitored, subsequently have defibrillation pulses applied, and later be monitored again all on the same electrodes. As disclosed in U.S. Pat. No. 3,547,108 issued Dec. 15, 1970 to Seiffert for Combination Defibrillator and Heart Monitoring System, a high voltage transfer relay is provided for isolating the monitoring equipment from the high voltage transient during delivery of a defibrillation pulse.
Transfer relays having the ability to withstand the high transient voltages associated with defibrillation pulses are quite expensive. Likewise, relays are mechanical in nature and subject to mechanical break down. As an alternative to such transfer relays, at least one combined defibrillation-monitoring system has employed an isolation amplifier at the input to the monitoring circuitry (that system being marketed by American Optical Corporation under the registered trademark PULSAR.RTM. 4). Such use of an isolation amplifier serves to substantially reduce the cost of circuit isolation as well as to permit some signal processing, such as filtering, not possible with a transfer relay. In this system there is also provided a separate set of patient electrodes and leads dedicated solely to the ECG monitoring function. That set of patient leads is selectively connected to the input of the isolation amplifier alternatively with the combined defibrillation-monitor leads via a lead selector switch. During defibrillation when high transient potentials appear on the defibrillation leads or other equipment leads, the air around the lead selector switch may break down and conduct, thereby damaging the switch. This occurs when a leakage circuit is created by a conductive foreign object at a different potential contacting either the leads or a floating-ground chassis in such as a battery powered system. Conventionally, neon tubes are used to protect such switches, however, they do not generally prevent these leakage circuits from occurring. Characteristically these tubes break down at a lower voltage (i.e. 80 volts) than the transient high voltages otherwise applied to the selector switch.
Accordingly, it is a principal object of the invention to provide an improved combination defibrillation and monitoring system of the type employing isolation amplifiers for circuit isolation and protection.